1. What name is given to the invention that uses the echoing or reflection of radio waves to detect objects or measure distances?

      Answer: Radar

2. Which of the following has usefully reflected radio waves from transmitters on Earth back to Earth: a) the ionosphere, b) the Moon, c) man-made satellites, d) Mars

      Answer: All of the above

MORE about Radar:

A normal radar instrument sends out short, high-intensity radio pulses and detects the reflections or echoes that are returned when the radio signals (i.e. the radio pulses) hit an object.

Radio waves travel at a constant speed - the speed of light, roughly 1,000 feet per microsecond - so, by measuring the time taken for the echo to be detected, the distance to the object from which the radio pulse was reflected can be calculated

Radar is used: to detect the presence of an object at a distance
to track planes on the ground and in the air
to guide planes in for smooth landings.
to detect the speed of passing vehicles.
to map the Earth and other planets,
to track satellites and space debris
to detect an enemy and to guide weapons.

MORE about Radio:

Radio waves are part of what we call the "electromagnetic spectrum" that is the result of energy emitted by hot objects such as our Sun. The complete range of electromagnetic radiation ranges from radio waves (long waves) to gamma rays (very short waves), including the visible (visible light) spectrum.

Waves in general are measured or specified by two properties: the physical length of the wave, and the number of times the wave cycle is repeated in a given period of time. Radio waves of different lengths are used for different purposes and are usually identified by their frequency (cycles per second).

In honor of Heinrich Hertz, the name Hertz has been given to the "cycles per second" measurement. For example, 1 kilohertz (kHz) is 1000 cycles per sec, 1 megahertz (MHz) is 1 million cycles per sec, and 1 gigahertz (GHz) is 1 billion cycles per sec. Radio waves range from a few kilohertz up to 30 gigahertz (30,000,000,000 Hz).

The shortest waves have the highest frequency, or number of cycles per second; the longest waves have the lowest frequency, or fewest cycles per second.

Radio Transmission

Radio waves emitted from a radio transmitter travel from the Earth to the ionosphere and are reflected back to the Earth.

The radio wave itself does not carry any information until it has had information superimposed on it. The addition of information to the radio wave (the "carrier" wave) is called modulation.

There are three common ways to modulate a radio wave.

Pulse Modulation - In PM, the radio wave is simply turned on and off. This is an easy way to send Morse code wirelessly over long distances.

Frequency Modulation - FM radio stations and hundreds of other wireless technologies (including the sound portion of a TV signal, cordless phones, cell phones, etc.) use frequency modulation. The advantage to FM is that it is largely immune to static. In FM, the radio transmitter's basic carrier radio wave frequency is overlain with the information signal frequency.

Amplitude Modulation - AM radio stations and the picture part of a TV signal use amplitude modulation to encode information onto radio waves. In amplitude modulation, the amplitude of the radio wave (its peak-to-peak voltage) changes. For example, the audio frequency wave produced by a person's voice is imposed onto the transmitter's carrier radio wave to vary its amplitude. An amplitude-modulated signal consists of the carrier frequency plus two sidebands resulting from the modulation.

The ionosphere is important for AM radio wave propagation.

• The ionosphere is composed of the D, E, and F layers
  
• The D layer is good at absorbing AM radio waves
  
• D layer disapears at night.... the E and F layers reflect the waves back to the earth
  

This is why radio stations adjust their power output at sunset and sunrise


Radio Reception

In radio reception a receiving aerial detects the waves sent out by a transmitter. A tuned circuit selects a particular frequency, usually by means of a variable capacitor. A demodulator separates the audio signal from the carrier, and an amplifier boosts the audio signal for feeding to the loudspeaker.


Some of the everyday technologies that depend on radio waves:

• AM and FM radio broadcasts
• Cordless phones
• Garage door openers
• Wireless networks
• Radio-controlled toys
• Television broadcasts
• Cell phones
• GPS receivers
• Ham radios
• Satellite communications
• Police radios
• Wireless clocks